Sabine Lisicki defended her decision to call a medical time-out at a decisive moment during her fourth-round victory over Yaroslava Shvedova. She received treatment at break point down and came back to secure a place in the quarter-finals against Simona Halep despite serving 20 double faults.

Halep and Lisicki will battle it out to join Lucie Safarova in the semi-finals, following the Czech’s victory over Ekaterina Makarova, and Petra Kvitova, who beat Barbora Zahlavova-Strycova. However, Lisicki’s 6-3, 3-6, 6-4 success was not without controversy.

The German, last year’s beaten finalist, was advantage down at 1-1 in the deciding set and appeared in severe discomfort, clutching her lower back and struggling to generate any power on serve. Lisicki signalled to the umpire that she could not continue without treatment, leaving Shvedova visibly frustrated.

The No19 seed received lengthy medical attention on her right shoulder before returning to action. She survived the break point and won the third set 6-4, although there were four breaks of serve before the end.

Lisicki won only 25% of points on her second serve and was regularly hitting tame efforts at around 60mph. Her count of 20 double faults is the third most in the history of ladies’ singles at Wimbledon.

“The timing was very unfortunate,” she said of the medical time-out. “I hit a ball before, and it just went into my back. I tried to keep going, I played a few points, but I wasn’t able to lift my arm. I mean it was obvious. I was serving, what, 50mph or something? I don’t remember if I ever served that slow in my life before. So that’s why I had to call the trainer; I just couldn’t lift my arm any more.

“I tried to play a few more points. I called before already for the trainer, then I played a few more points and I just wasn’t able to. You know how I serve. That was not nearly what I could do. I just couldn’t lift my arm.” Shvedova’s rhythm seemed disrupted in the final set and the Kazakh conceded three match points at 5-4 down, netting at 40-30 and prompting Lisicki to crumple to the floor.

The Australian coach Darren Cahill, who has worked with Andy Murray, tweeted: “Just for the record, if you need to call a trainer mid game, no matter what the score, you should be forced to forfeit that game, imo. Sabine is playing within the rules but we need to address them. Unfair to opponents. If the injury needs immediate attention, lose that game.”

Lisicki added: “I just know I’m honest. I really tried to play several more points, that’s what I tried. I have a big respect for my opponent. It’s the first time that I had to do this ever. I have a huge respect for her – we played a tight match before. I feel lucky that I got away with it, with serving 60mph serves. I think it was very visible.”

Lisicki faces Halep for the first time in a grand slam, the Romanian taking just 57 minutes to overcome Zarina Diyas of Kazakhstan. Halep was in fine form to dispatch Diyas 6-3, 6-0, edging the opening set before steamrolling her young opponent in the second.

Diyas, 20, stayed in touch early on but became increasingly frustrated as the match progressed. The promising world No72 was comprehensively outplayed in the second set and regularly looked to her coaches in frustration.

Safarova, the No23 seed, took 58 minutes to beat Makarova 6-3, 6-1, with the Czech’s serve too strong for the Russian on Court One. It is her best performance in a grand slam, after reaching the quarter-finals of the Australian Open in 2007 and the last 16 at Roland Garros this year.